1. Field of the Invention
This invention relates to the development of nickel based, boron-free, metal brazing alloys having liquidus temperatures in the range of 877.degree.-948.degree. C. (1611.degree.-1738.degree. F.).
2. Description of the Prior Art
Brazing is a process for joining metal parts, often of dissimilar composition, to each other. Typically, a filler metal that has a melting point lower than that of the base metal parts to be joined together is interposed between the metal parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the filler metal. Upon cooling, a strong, corrosion resistant, leak-tight joint is formed.
Certain steels and superalloys used in construction of nuclear reactor components require brazing filler metals capable of providing high joint strengths. In these applications, the filler metals cannot contain boron, due to the boron atom's relatively high cross section for neutron absorption with consequent reduction in reactor efficiency. It has therefore been proposed that nickel based filler metals containing silicon or phosphorous as melting point depressors, be used in such applications. The brazing temperatures of the silicon containing filler metals are extremely high. A typical silicon containing filler metal having the American Welding Society designation (AWS) BNi-5 has a brazing temperature of about 1149.degree.-1204.degree. C. (2100.degree.-2200.degree. F.). Exposure to temperatures of this magnitude during brazing deteriorates the mechanical properties of many steels and superalloys, rendering them unsuitable for use in construction of nuclear reactor components. The brazing temperatures of phosphorous containing filler metals are somewhat lower than their silicon containing counterparts. For example, the brazing temperatures of phosphorous containing filler metals designated AWS BNi-6 and BNi-7 are about 927.degree.-1093.degree. C. (1700.degree.-2000.degree. F.). However, the phosphorous containing filler metals produce brittle joints which exhibit poor joint strengths.
Therefore, there remains a need in the art for a nickel based brazing alloy that is free of boron and phosphorous and is adapted for brazing at temperatures less than 1000.degree. C.